2 subs just sounds more full to me than a single sub. yes a single sub can get loud but for some reason having 2 subs just seem to make the bass more full.
i know some of you might say "chad, my single sub setup sounds really full and loud. also clean."
well go add another one the same as you have and you'll know what i am talking about.
I would say for the single sub set up being louder than a dual is skewed as 99% of the time it rests on the box design, and some people make wicked boxes.
With 3,000rms and 2 12" in a trunk I was hitting 147db. Next guy had a little Sprint with a single 12" sub, some weird aero ported box and 6krms and was hitting 154db. As for his set up, I'm willing to bet it sounds trashy on music.
In a similar set up with same subs, car and power and not some fancy box the 2 subs will come out on top.
One sub is more impressive when it produces massive output.
In practical terms though, more subs are better if you have the space. Take Rays case for example. By running 2 SX at half power instead of 1 at full power, he will get the same SPL, but run no risk of overheating the coils, even if his amp clips a little. (which often happens in comp. when voltage starts to drop).
lol space usually aint no diff,compare 2 sx 12 to one sx 15,not much space diff....i just want one 15 to trip ppl out when its louder than there pair lol,,my town is noobs,even one car audio shop has ppl worst than noobs running it here